1. Field of the Invention
The present invention relates generally to a combustion-type power tool, such as combustion-type nail driver for striking fastening members such as nails or studs into a workpiece wherein acceleration applied to a motor when combustion explosion occurs or a piston impinges upon a bumper is suppressed.
2. Description of the Related Art
A combustion-type nail driver ignites air-fuel mixed gas confined in a combustion chamber and translates voluminal expansion of the gas into power. A fan is disposed within the combustion chamber to stir air and fuel to enhance the combustion property of the mixed gas.
The fan is rotated by a motor. The fan generates turbulence of the mixed gas in the combustion chamber and promotes combustion of the gas. Occurrence of explosive combustion in the combustion chamber brings the voluminal expansion of the gas and generates impact. The impact thus generated is transmitted to the body of the nail driver, and so to the motor for rotating the fan.
A piston that translates the voluminal expansion of the gas into power strikes a nail into a workpiece. A kinetic energy in excess of the energy required for striking the nail into the workpiece is absorbed into a bumper disposed in the cylinder along which the piston slidingly moves when the piston impinges upon the bumper. At this time, acceleration generated when the piston impinges upon the bumper is applied to the body of the nail driver, and the acceleration thus generates is transmitted to the motor.
Because the motor is a precise device and is weak against vibration. The motor may be damaged by the impact repeatedly applied to the motor, resulting in degradation of the property of the motor. In order not to transmit the impact to the motor, a buffer material is used for a motor holding member. The motor holding member separates the motor from the body of the nail driver, thus transmission of the impact to the motor can be prevented, as disclosed in U.S. Pat. No. 6,520,397.
More specifically, as shown in FIGS. 1 and 2, a motor 118 is mounted on a cylinder head 111. The cylinder head 111 is disposed at one end portion of a housing 102 of the nail driver 101 and covered by a head cover 103. A fan 119 is attached to the tip end of the output shaft 118b of the motor 118. A spark plug 112 fixedly secured to the cylinder head 111 is positioned in the vicinity of the motor 118 and has one end projected into the combustion chamber.
Two circumferentially extending grooves are formed over the entire outer periphery of the motor 118 to be spaced apart in the axial direction of the motor 118. As shown in FIG. 1, a retaining ring 114 is fitted into each of the two grooves. An inner ring 113a of a buffer member 113 is interposed and held between the two retaining rings 114.
As shown in FIG. 2, the buffer member 113 includes the inner ring 113a, a fixing metal member 113c, and a rubber member 113b molded and coupled to both the inner ring 113b and the fixing metal member 113c to be integral thererwith. The fixing metal member 113c is fixedly secured to the cylinder head 111. As such, the motor 118 is supported on the cylinder head 111 is via the buffer member 113.
With the above-described structure, impact generated in the nail driver 101 is transmitted to the fixing metal member 113c of the buffer member 113. However, due to the presence of the rubber member 113b, the impact transmitted to the inner ring 113a and to the motor 118 supported by holding the inner ring 113 with the retaining rings 114 is suppressed.
However, as described above, with the conventional nail driver, the grooves need to be formed in the outer periphery of the motor 118 in order to fix the motor 118 to the buffer member 113. Therefore, general-purpose motors cannot be employed but motors manufactured based on a special specification, which are expensive in cost, are required. The buffer member 113 is an integral member in which the two metal rings 113a and 113c are connected together with the rubber member 113b interposed therebetween. Due to the different materials forming the integrated buffer member 113, reliability of rubber mold coupling is low and there is a possibility that the different material segments are separated if the rubber molding condition is not good.
From the structural requirement, the spark plug 112 is positioned in the vicinity of the motor. Accordingly, the rubber member 113 cannot extend to the position of the spark plug 112. Continuity of the rubber member 113 is thus interrupted by the spark plug 112 and the rubber member 113 is separated at the position of the spark plug 112. The buffer member 113 is incapable of equally suppressing the impact to be imparted upon the motor 118. Tensile stress is thus focused on a position near the spark plug securing position, so that the rubber member 113b is liable to be damaged.
In the nail driver of the type described above, continuously performed nail driving operations accumulate heat generated at the time of explosive combustion. The combustion chamber 26 and the cylinder (not shown) are the primary sources of heat generation. The heat thus generated is transmitted to and raises the temperature of the nail driver including the motor 118. Driving the motor 118 also generates heat in the motor coil, so further raises the temperature of the motor 118. A problem that temperature rise of the motor 118 may cause burning of the motor 118 has conventionally been solved by employing highly durable motors. However, such motors are expensive in cost.